Adsorption of β-glucosidases in two commercial preparations onto pretreated biomass and lignin

Background: Enzyme recycling is a method to reduce the production costs for advanced bioethanol by lowering the overall use of enzymes. Commercial cellulase preparations consist of many different enzymes that are important for efficient and complete cellulose (and hemicellulose) hydrolysis. This abundance of different activities complicates enzyme recycling since the individual enzymes behave differently in the process. Previously, the general perception was that beta-glucosidases could easily be recycled via the liquid phase, as they have mostly been observed not to adsorb to pretreated biomass or only adsorb to a minor extent. Results: The results from this study with Cellic (R) CTec2 revealed that the vast majority of the beta-glucosidase activity was lost from the liquid phase and was adsorbed to the residual biomass during hydrolysis and fermentation. Adsorption studies with beta-glucosidases in two commercial preparations (Novozym 188 and Cellic (R) CTec2) to substrates mimicking the components in pretreated wheat straw revealed that the Aspergillus niger beta-glucosidase in Novozym 188 did not adsorb significantly to any of the components in pretreated wheat straw, whereas the beta-glucosidase in Cellic (R) CTec2 adsorbed strongly to lignin. The extent of adsorption of beta-glucosidase from Cellic (R) CTec2 was affected by both type of biomass and pretreatment method. With approximately 65% of the beta-glucosidases from Cellic (R) CTec2 adsorbed onto lignin from pretreated wheat straw, the activity of the beta-glucosidases in the slurry decreased by only 15%. This demonstrated that some enzyme remained active despite being bound. It was possible to reduce the adsorption of Cellic (R) CTec2 beta-glucosidase to lignin from pretreated wheat straw by addition of bovine serum albumin or poly(ethylene glycol). Conclusions: Contrary to the beta-glucosidases in Novozym 188, the beta-glucosidases in Cellic (R) CTec2 adsorb significantly to lignin. The lignin adsorption observed for Cellic (R) CTec2 is usually not a problem during hydrolysis and fermentation since most of the catalytic activity is retained. However, adsorption of beta-glucosidases to lignin may prove to be a problem when trying to recycle enzymes in the production of advanced bioethanol.